The invention relates to a catalyst comprising (and preferably constituted by) delta alumina, rhenium and caesium, and to a process for preparing the catalyst. The invention also relates to a metathesis process using this catalyst, and more particularly to a process for metathesis of an olefinic C4 cut (i e., containing butenes).
In one particular aspect, the present invention relates to a process for converting a C4 hydrocarbon cut containing isobutene, after selective hydrogenation of butadiene and acetylenic compounds and hydroisomerisation of 1-butene to 2-butene, by metathesis with ethylene in the presence of a catalyst based on rhenium improved by the incorporation of caesium and by the presence of delta alumina.
Cracking light paraffins produces ethylene and propylene required for petrochemistry. It also produces a certain number of other products including a C4 hydrocarbon cut which principally contains butadiene, isobutene, n-butenes and butanes, also traces of acetylenic hydrocarbons.
The metathesis reaction converts such cuts, which are often inadequately used, to compounds with a higher intrinsic value. As an example, metathesis with ethylene of an olefinic C4 cut, which has already undergone selective hydrogenation of butadiene and acetylenic compounds and hydroisomerisation of 1-butene to 2-butene, can produce propylene.
When such a reaction is carried out using the usual rhenium-based metathesis catalysts, with a C4 cut containing isobutene, this partially polymerises on contact with the catalyst and that secondary reaction causes a large reduction in the duration of the cycle for the catalyst between two regeneration operations.
Incorporating caesium into a rhenium-based metathesis catalyst has been described in French patent FR-A-2 373 504, for the preparation of branched olefins. In U.S. Pat. No. 5,057,644, caesium has been cited as a possible additional alkaline element in a catalyst activated at 300-600xc2x0 C. brought into the presence of organic borane as a promoter. FR-A-1 572 314 indicates that the selectivity of a metathesis catalyst can be increased by adding caesium in particular, and that activation occurs at 300-750xc2x0 C.
The alumina used as a support in the prior art is generally a gamma alumina and the catalyst is heat activated in a temperature range which usually does not exceed 750xc2x0 C., although U.S. Pat. No. 3,594,440 indicates that it is possible to go up to 900xc2x0 C., with no mention of a particular advantage. In a more recent publication, it is indicated that there is no interest in activating a caesium-rhenium-alumina catalyst at a temperature of more than 300xc2x0 C., the catalyst being prepared by impregnating rhenium into alumina, calcining at 550xc2x0 C., introducing caesium by impregnation, calcining at 500xc2x0 C., then activating in nitrogen (T. Kawai et al., Journal of Molecular Catalysis, vol. 76, pp. 249-261, 1992).
We have now discovered that the use of a rhenium- and caesium-based catalyst which prior to the reaction has been treated at a temperature of more than 750xc2x0 C., generally at most 1000xc2x0 C., leads to a substantial reduction in the deactivation rate, without substantially affecting the activity for the metathesis reaction, for example, which substantially increases the duration of cycles between two regeneration steps.
Without wishing to be bound by a particular interpretation, heat activation of a gamma alumina at temperatures beyond 750xc2x0 C., and generally of at most 1000xc2x0 C., is known to partially transform it into delta alumina. The beneficial effect on performance could be considered to be attributed to the presence of this type of alumina combined with the presence of caesium.
The catalyst used in the process of the invention thus comprises (and is preferably constituted by) at least three components:
a porous alumina-based support; more generally, the support is principally composed by alumina, and advantageously it contains at least 75% by weight of alumina; preferably it is constituted by alumina, with at least a portion of the alumina being delta alumina (at least 0.5% by weight and preferably at least 1% by weight, or more preferably, 5% by weight and preferably 5-50% by weight)
0.01% to 20% by weight of rhenium;
and 0.01% to 5% by weight of caesium.
The present invention also concerns a process for preparing said catalyst. In this process, a catalyst precursor based on gamma alumina and rhenium is formed and said precursor undergoes heat treatment at more than 750xc2x0 C. in a non reducing gas atmosphere. In one implementation, the precursor also contains caesium. In a preferred implementation, the precursor containing rhenium but not caesium, which has been heat treated at more than 750xc2x0 C., is impregnated with a caesium compound, dried than activated.
The porous starting support is based on gamma alumina, and preferably contains at least 75% by weight of alumina, and advantageously has an appreciable surface area, for example at least 10 m2/g, and preferably at least 50 m2/g, and a sufficient pore volume, for example at least 0.1 ml/g, preferably 0.3-1 ml/g.
The rhenium compound can be introduced into the support, for example by vapour phase sublimation or by impregnation in solution. In general, the dry impregnation method is preferably used, where the rhenium compound is dissolved in water or in an organic solvent, for example a hydrocarbon, an alcohol or an ether. The quantity of rhenium on the support is adjusted by selecting the concentration of the impregnating solution. When the quantity of rhenium which is to impregnated is higher than that which a solution at its saturation limit will allow, the operation must be carried out several times with intermediate drying steps to eliminate the impregnation solvent, at a temperature of 90xc2x0 C. to 250xc2x0 C., for example, preferably 100xc2x0 C. to 180xc2x0 C. This enables 0.01% 20%, preferably 0.1% to 15%, more advantageously 0.5% to 8% by weight of metallic rhenium, to be introduced. Preferred rhenium compounds are rhenium heptoxide, ammonium perrhenate and perrhenic acid.
After the rhenium impregnation step, a catalyst precursor is obtained, then drying is carried out at a temperature of 90xc2x0 C. to 250xc2x0 C., for example, preferably 100xc2x0 C. to 180xc2x0 C., followed by calcining at a temperature of more than 750xc2x0 C. and advantageously at most 1000xc2x0 C., preferably more than 800xc2x0 C. and advantageously at most 900xc2x0 C., in a non reducing gas atmosphere, for example oxygen, nitrogen or argon, oxygen diluted with nitrogen, preferably in air, under static or dynamic conditions, a slow gaseous stream being preferable, however. The amount of moisture in the gaseous stream is preferably kept below 200 ppm (parts per million). However, it is possible to heat in an atmosphere constituted by methane combustion gases or a natural gas in the presence of an excess of air. The duration of this activation treatment is, for example, from 10 minutes to 5 hours or more, after which the precursor obtained is cooled in an atmosphere which is preferably anhydrous. During this calcining treatment, a portion of the gamma alumina is transformed into delta alumina, for example at least 0.5% by weight, preferably 5% to 50% by weight of delta alumina with respect to the starting alumina.
The caesium compound can be introduced into the support using any of the usual methods used in heterogeneous catalysis, for example by solution impregnation. In general, it is preferable to use the dry impregnation method, described above. The caesium compound is dissolved in water. The volume of the solution is less than or at a maximum equal to the volume of the pores of the support. The quantity of caesium on the support is adjusted by selecting the concentration of the impregnation solution. When the quantity which is to be impregnated is higher than that which can be introduced by a solution at its saturation limit, the operation must be carried out several times, with intermediate drying steps to eliminate the impregnation solvent, at a temperature of 90xc2x0 C. to 250xc2x0 C., for example, preferably 100xc2x0 C. to 180xc2x0 C. This enables 0.01% to 5%, preferably 0.1% to 3%, and more advantageously 0.2% to 2% by weight of metallic caesium to be introduced. The caesium compound is advantageously a salt such as a halide or a sulphate, preferably a nitrate.
The catalytic composition obtained following the preceding steps is activated by heating between 400xc2x0 C. and 1000xc2x0 C., preferably between 500xc2x0 C. and 900xc2x0 C., and more preferably between 400xc2x0 C. and 600xc2x0 C. If the temperature is increased beyond 750xc2x0 C., delta alumina formation is observed, but also rhenium is lost due, it appears, to the presence of the caesium. This heating is carried out in a non reducing gas atmosphere, for example: oxygen, nitrogen or argon, oxygen diluted with nitrogen, preferably in air, under static or dynamic conditions, a slow stream of gas being preferable, however The amount of moisture of the gas stream is preferably kept below 200 ppm (parts per million). The duration of the activation treatment is, for example, 10 minutes to 5 hours or more, after which the active catalyst obtained is cooled in an atmosphere which is preferably anhydrous. Advantageously, a nitrogen purge is carried out, if necessary, before bringing it into contact with the hydrocarbon-containing feed.
The present invention also concerns a metathesis process using the catalyst described above or obtained by the preparation process described above
More particularly, the present invention concerns a process for metathesis of an olefinic C4 cut containing 2-butene to produce propylene and isobutene.
When said cut contains diolefins, 1-butene, 2-butene, isobutene and acetylenic impurities, said process comprises, prior to metathesis:
1) selective hydrogenation of the diolefins and acetylenic impurities with isomerisation of 1-butene to 2-butene, to obtain an effluent containing mainly 2-butene and isobutene, and containing practically no diolefins or acetylenic compounds;
2) then separation, by distillation, of an overhead cut containing mainly isobutene and 1-butene not converted in the first step, and a bottom cut containing essentially 2-butene and butane as well as a little residual isobutene;
metathesis being followed by separation of the propylene.
Thus the present invention proposes a process for treating a C4 hydrocarbon cut containing principally isobutene, n-butenes, butanes, 1,3-butadiene in varying quantities, which includes selective hydrogenation of the diolefins and acetylenic impurities with isomerisation of 1-butene to 2-butene, separation of the isobutene by distillation, and metathesis of the 2-butene cut from the preceding step with ethylene, which transforms the 1,3-butadiene and n-butenes to propylene which can, for example, be used for polymerisation. In the metathesis step, the use of a rhenium catalyst modified by caesium enables a relatively high residual isobutene content to be tolerated in the 2-butene cut, and thus the severity of the isobutene distillation can be reduced.
The preferred process of the invention is more precisely a process for converting an olefinic C4 cut to isobutene and propylene, said cut containing diolefins, 1-butene, 2-butene, isobutene and acetylenic impurities, said process comprising the following steps carried out successively:
1) selective hydrogenation of the diolefins and acetylenic impurities with isomerisation of 1-butene to 2-butene, to obtain an effluent containing mainly 2-butene and isobutene, and containing practically no diolefins or acetylenic compounds, preferably by passing said cut in the liquid phase over a catalyst comprising at least one metal selected from the group formed by nickel, palladium and platinum, deposited on a support, at a temperature of 20-200xc2x0 C., a pressure of 0.1-5 MPa, a space velocity of 0 5-10 hxe2x88x921, with an H2/diolefins ratio (molar) of 0.5 to 5 and preferably 1 to 3;
2) separation, by distillation, of an overhead cut containing mainly isobutene and 1-butene not converted in the first step, and a bottom cut containing essentially 2-butene and butane as well as a little residual isobutene;
3) metathesis of the 2-butene cut from the preceding step with ethylene, in the presence of a catalyst of the invention as described above, the catalyst containing delta alumina, rhenium and caesium (catalyst preferably having undergone a treatment at a temperature of more than 750xc2x0 C. and at most 1000xc2x0 C. to transform a portion of the alumina to delta alumina), at a temperature in the range 0 to 100xc2x0 C., and at a pressure at least equal to the vapour tension of the reaction mixture at the reaction temperature, so as to obtain an effluent containing propylene, metathesis being followed by separation of propylene.
In the process of the invention, the metathesis reaction is preferably carried out in the liquid phase, in the absence of oxygen, oxygen-containing compounds, nitrogen-containing compounds or sulphur-containing compounds and moisture, at a temperature in the range 0xc2x0 C. to 100xc2x0 C., preferably in the range 30xc2x0 C. to 100xc2x0 C., and at a pressure of at least the vapour tension of the reaction mixture at in the reaction temperature.
The catalyst can be used in a fixed bed. In this case, as it has to be regenerated frequently, it is then necessary to provide at least two reactors in parallel, one being in operation while the other is being regenerated. It is also possible to use a moving catalytic bed system such as that described in French patent FR-A-2 608 595. The catalyst is extracted at regular time intervals from the bottom of the reactor and transferred to a continuous regeneration system, from which the regenerated catalyst is returned to the top of the reactor.
Because of the limits imposed by thermodynamics, the unconverted reactants are fractionated and can be recycled to the metathesis reactor.
The following example illustrates the invention without limiting its scope.